In FIG. 1, prior art assemblies are illustrated where electrical connection to a transducer 1 is made using, in a), three wires 2 each soldered to solder bumps 3 of the transducer.
The set-up of a) has the disadvantages that it is difficult to automate and that rather large solder bumps or soldering pads are required, thus requiring a large amount of heating.
In b), the conductors 2 form part of a flex print 2′ having openings 4 each surrounded by a part of the pertaining conductor 2, and the transducer 1 has solder bumps 3. Fixing of the transducer 1 to the flex print 2′ is performed by attempting to position the flex print 2′ in relation to the transducer 1 so that the solder bumps 3 and the openings 4 are aligned.
Because the solder bumps 3 are larger than the holes 4 and everything, when the transducer 1 is e.g. intended for use in hearing aids, is too small to add optical targets (so-called “fiducials”), the positioning is presently done either based on the outside lines of the flex 4 and transducer (suffering from a large tolerance build-up) or based on looking through the flex-holes 4 (which is actually ‘blind’ once the holes are over the larger solder bumps).
The transducer 1 and flex print 2′ must then be fixed (in three dimensions) while soldering is performed.
Especially in lead-free systems, flux-less soldering is problematic, whereby it is preferred to add some flux, solder paste or flux-core solder. Also, for sufficient solder-wetting, it is preferred that the applied flux sufficiently contacts the solder bump as well as the flex hole contact area. Since the flex is preferably rather thick compared to the hole-size (relatively deep hole), it is difficult to dispense the flux correctly. Checking visually after dispensing is also difficult, as the flex is positioned over the solder bumps. This check is, however, desired, as flux is not allowed to enter on to the surface between the solder bumps 3, as this brings about a risk of short-circuit and corrosion.
Having added the flux, the soldering is to be performed. This soldering is performed through the flex print 2′, as the solder bumps 3 are larger than the openings 4, and must comprise heating sufficiently to melt the solder bumps 3. This heating may bring about a number of problems, such as damage to the transducer, melting and short circuiting of the solder bumps 3, damage to the flex print, etc. Furthermore, there is a considerable risk of partial melting and bad wetting due to inhomogeneous heat transfer, bringing about an insufficient solder connection quality.
A further complication is that, during soldering, the flex should be kept in the desired position, but a pressed-down flex mounted on this type of solder bumps will usually slant or bend as all solder bumps are not melted simultaneously, but consecutively due to the process of applying heat. As the flex print has three degrees of movement as indicated by the coordinate system in FIG. 1b, it may slant in any of the directions indicated.
Finally, it is noted that the connection built with flex and the solder bumps 3 has the flex cover a large part of the solder bumps 3 and the space between the bumps 3. Thus, visual inspection is made quite difficult, whereby there is a considerable risk that a low wetting quality is not spotted, and that short circuits between bumps 3 and flux residues between bumps 3 goes unnoticed (unused flux is etching and causes corrosion risks in the field and burned flux causes a risk for brittle solder connections).